Method and system for the electric determination of aerosols in a gas



Aug. 1, 1961 DERFLER 2,994,768

METHOD AND SYSTEM FOR THE ELECTRIC DETERMINATION OF AEROSOLS IN A GAS Filed Jan. 23, 1958 United, StatesPatnt O METHOD AND SYSTEM FOR THE ELECTRIC DETERMINATION OF AEROSOLS IN A GAS Heinrich Dertler, Bad-Ragaz, Switzerland, assignor to Cerberus G.m.b.H., Bad-Ragaz, Switzerland, a company of Switzerland Filed Jan. 23, 1958, Ser. No. 710,732 Claims priority, application Switzerland Jan. 25, 1957 9 Claims. (Cl. 250-435) The present invention relates to a method of checking gases for their aerosol content and to a device for the performance of the said method. For the purpose of the present specification, the term aerosol shall be deemed to mean particles of sub-microscopic to microscopic magnitude suspended in gases, e.g. in air. Aerosols are produced, by way of example, in many chemical reactions such as most combustion processes. The method and the device according to this invention are, therefore, suitable for the determination of smoke and combustion gases, and in particular for automatic fire alarms.

The use of ionization chambers for the electrical control of the composition of gases has long been known. Although one embodiment of the present invention employs an ionization chamber, the invention is distinguished from the known arrangements since a fundamentally different physical process is employed. Owing to the difierent operation, the arrangement and the design of the ionization chamber employed is different from those of conventional use.

It is a primary object of the present invention to provide a safe and reliable means of detection of aerosols in a gas.

It is a further object of the present invention to provide a safe and reliable means of detection of aerosols characterized by extremely high sensitivity.

It is a further object of the invention to provide a method for determining the presence and the content of aerosols in a gas by means of measuring the current flowing in a gas discharge device.

It is another object of the present invention to provide a system for determining the content of aerosols in a gas by means of measuring a unipolar current flowing in a gas discharge device.

A further object of the invention is to provide means in a gas discharge device for effecting the occurrence of charged particles of only one polarity in at least a portion of said gas discharge device.

It is another object of the invention to provide a safe, sensitive and reliable fire alarm device, which is actuated by aerosols occurring as a product of combustion processes.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawings in which:

"FIG. 1 is a schematic view of a known device for the determination of aerosols in a gas;

FIG. 2 a graph showing the characteristic of the device according to FIG. 1;

FIG. 3 a schematic view of a device for the determination of aerosols in a gas according to one embodiment "of the invention for determining aerosols in a gas;

FIG. 7 a diagram of a fire alarm device according to the invention, and

FIG. 8 the structure of the device according to FIG. 7. In order to explain this difference, the processes tak- Ice ing place within an ionization chamber must be discussed in greater detail.

The conventional method will now be described in conjunction with FIG. 1. Applied to the plates 1 and 2 of an ionization chamber is an electric voltage which is smaller than the saturation voltage. The entire space between the plates is ionized by the source 3 of radioactive radiation. As can be seen from the drawing, the radiation from the source 3 can reach every point of the space between the electrodes 1 and 2. The diagrammatically represented charged particles and ions respectively travel in the direction indicated so that an electric current flows through the chamber of which the magnitude is determined by the intensity of the radiation source, the geometry of the arrangement and the magnitude of the voltage applied. The dependency on the voltage of the current results from the fact that with medium voltages not all ions produced do reach the electrodes but vanish owing to recombination with charged particles of the other polarity. In FIG. 2 curve 8 shows the course of the current I diagrammatically as a function of the voltage V for pure air. If aerosols, i.e. particles much larger than gas molecules, reach the space between plate 1 and 2, the recombination conditions change. Part of the ions accumulate on aerosol particles so that the rate of travel i.e. the velocity of the relative ions is substantially reduced. These heavy ions remain in the gap much longer so that the likelihood of a recombination process is correspondingly greater owing to the extended dwelling time. The increased recombination in the presence of aerosols, therefore, causes a reduction of the ionization flow at unchanged voltages. An example of the connection between current and voltage in the presence of aerosols is shown by curve 9 in FIG. 2.

The essential features of the process so far known are, therefore, the presence of ions of both polarities in the measuring space (bipolar ion flow) and an increased likelihood of recombination by accumulation of ions on aerosol particles. Against this, a substantially larger alteration of the flow in the presence of aerosols can be obtained in the process according to this invention. The method according to this invention is characterized by the fact that charged particles of a single polarity are introduced at least into a portion of the space delimited by the electrodes, so that a unipolar current will be set up in at least said portion of the space.

The method according to this invention will now be discussed in conjunction with the embodiment shown in FIG. 3. Again, 1 and 2 are electrodes to which a voltage is applied with the negative pole at electrode 1; 3 designates a radioactive radiation source e.g. a radiocative substance accommodated in a unilaterally open screening container serving as a shield means which ensures that the radiation can reach only the portion 4 of the space defined by the two electrodes 1 and 2. The portion 4 as can be seen in the drawing is smaller than the remaining portion 5 of the space between the electrodes. As further can be seen, the portion 4 i.e. the portion which is radiated by the radioactive source is smaller than the remaining portion 5. Ions of either charge are pro duced in the space 4 and travel in the direction indicated. Part of the negative ions very quickly leave the ionization area 4 and move through the space 5 not under the action of radiation towards the anode 2.

While ions of both signs travel in the space 4 (bipolar current), the space 5 holds only negative ions (unipolar current) so that a negative space charge is created. The portion 4 of the space therefore serves to deliver charged particles of only one polarity. The particles of the one polarity alter the static field between the electrodes 1 and 2 and, in particular, the electric field strength in the border area between the portions 4 and 5 which deterv If 2,994,768 is p A mines the flowing cur-rent. The resulting field intensity i in the border area naturally depends on the overall voltage V applied to the electrodes 1 and 2, i.e. the higher the voltage the more ions are moved into space 5 from space 4. The connection between current I'and voltage V for pure air is shown as the curve 11 in FIG. 4. If aerosols enter the space between the electrodes 1 and 2, part of the ions will again accumulate on the aerosol particles. The increased recombination mentioned above only occurs in the small space 4 of the bipolar ion flow. The significance of this process is, however, small since, within certain limits, the flowing total current little depends from the ion density in the space portion 4. Owing to the presence of ions of a single polarity, no recombination is possible in the space portion 5. The flowing unipolar current, therefore, largely consists of heavy ions of a single polarity where the aerosol concentration is high. Owing to their insignificant mobility and the impossibility of recombination, these particles remain in the field for a comparatively extended period so that the field condition corresponding to equilibrium is reached with a much lesser ion flow, i.e. at the same exterior voltage, a much smaller current flows in the presence of aerosols than in their absence. These conditions are represented by curve 12 in FIG. 4. The phenomenon may also be described as follows: In order to obtain the same current 1 as in the absence of aerosols, the voltage V must be increased by AV to the value V Experience shows that the said voltage increase AV is approximately proportionate to the number of aerosol particles present in the gas per unit volume.

FIG. 5 shows a further embodiment of the invention. Arranged in a container 23 provided with perforations is an electron emitting electrode, e.g. a hot cathode 21 which is suppliedby a voltage source 24. Provided opposite this cathode is an anode 22 connected with the said cathode via a voltage source 25 and a current measuring instrument 26. The voltage source 25 is so dimensioned that the flowing ion current is not saturated in the absence of aerosols. if aerosols enter the container 23 through its perforations, the electrons emitted by the hot cathode 21 accumulate on the aerosols and thus form heavy ions which move towards the anode substantially less rapidly and, therefore, tend towards a condition of equilibrium with lesser currents. A reduction of voltage is, therefore, indicated by the current measuring instrument 26 as a consequence of the entry of aerosols.

The advantages of the process according to this invention over the known methods are these: With the same aerosol concentration much larger current variations can be obtained. In practical arrangements the process according to this invention has resulted in a relative current variation larger by the factor 3 to than the current variation obtainable with the said known arrangements.

Thanks to the increased sensitivity, the current variations caused by climatic fluctuations of pressure, temperature and humidity are relatively less important. It has further been found that the action of draughts on the current is as such much smaller in the new process and can be additionally reduced by a suitable design of the electrodes.

FIG. 6 shows an embodiment of the arrangement for the performance of the method. The reference numeral 31 indicates a jacket-type electrode which is permeable for aerosols. It may, by way of example, be designed as a fine-mesh screen or it may be provided with perforations. The counterelectrode is designed as a centrally positioned rod 32. Attached to the electrode 31 are shield means in the form of a ring 33 the outside of which is covered with a radioactive substance, so that the portion of egg. jlhooc radiation effective in respect of ionization can become operative only in a portion of the 4 7 space between the electrodes. This space portion, which is in the vicinity of the electrode 31 corresponds with the space portion 4 of the embodiment shown in FIG. 3.

In all cases discussed, the polarity of the voltage and of the charge carriers may be reversed.

The process according to this invention and the device respectively may be employed for a single or continuous measurement or registration of aerosol concentrations. The chamber disclosed may, by way of example, be connected to a voltage source in series with a resistance, the potential variation at the two electrodes being a measure of the aerosol concentration and serving, if desired, to actuate an indicator member. It is evident that the potential variation reaches an optimum when the resistance displays saturation characteristic, i.e. possesses infinite resistance in the operative range. A second ionization chamber may be'employed as a resistance in which radioactive radiation occurs as well.

It is advantageous to employ a cold three-electrode or gas filled discharge tube as the indicator member, the control circuit being connected in parallel with the measuring chamber.

FIG. 7 shows a complete circuit of a fire alarm. The measuring chamber 34consists of the electrodes 31 and 32 and the radiation source 33 similar to the arrange ment shown in FIG. 6. The chamber 34 is connected in series with the comparator chamber 35 operating in saturation, the latter comprising the cathode 36, 'the anode 37 and the radiation source 38. Arranged parallel with these two ionization chambers is the ionical relay or cold cathode tube 39 with a cathode 40, a control electrode 41 and an anode 42. This arrangement is connected, via the coil of relay 43, to the voltage source 44. The contact of the relay 43 is arranged in the circuit of an alarm system consisting of a battery 16 and a horn 17. If combustion gases enter the ionization chamber 44, the voltage at the control electrode 41 will rise and ignite the cold cathode tube 39. A strong current will then flow through the coil of relay 43 so that the alarm circuit is closed through contact 45.

The structural design of such an arrangement is shown in- FIG. 8. The reference numerals 3042 in FIG. 8 designate the same members as in FIG. 7. Attached to a base 48 is a housing 51 which encloses the ionization chamber 35. Attached to the said base is the cold cathode tube 39 which carries the ionization chamber 35 on the portion projecting from the base 48. The control electrode 41 is arranged on the wall of the ionization chamber 35 facing the cold cathode tube 39 and it projects into the interior of the cold cathode tube 39 A as shown. A contact pin '49 is connected to the cathode 40 and the housing 51, and a contact pin 50 with the anode 42 and the electrode 37 of the ionization chamber 35. The electrode 37 is thus placed in the interior of this chamber 35. l

The ionization chamber 34 has an outer. electrode 31 designed as a perforated cap which may be attached to the housing 51. The electrode 32 of chamber 34 projects from the outer wall of chamber 35. The radioactive substance 33 is arranged similarly to that disclosed in connection with FIG. 6;

Other embodiments are possible besides those disclosed. It is, by way of example, possible to employ a radiation source common to both ionization chambers, the separation of the two chambers being effected by a sheet permeable to radiation. Also it may not be necessary to separate the two chambers hermetically, e.g. a single chamber may be divided into two sections by an intermediate screen so that the one essentially has a bipolar saturation cnrrent while a unipolar current flows inthe other. The action of aerosols on a chamber operating in saturation is so small that it may practically be disregarded in view of the variation in the unipolar chamber.

Again, means for the alteration of the measuring range may be'provided as well. These means may consist of devices for the geometric alteration of the chamber or in devices for the alteration of the position of the radioactive radiation source, or in an adjustable or interchangeable stop in front of the radiation source.

What I claim is:

l. A method for determining the presence of aerosols in a gas comprising the steps of generating an electrostatic field, subjecting said gas to said electrostatic field, injecting ionizing rays into a portion of said electrostatic field to thereby generate charged particles in said portion of said field and efiect the presence of charged particles of only one polarity in said remaining portion of said field, measuring the current flowing in said field, and comparing said current with that flowing through a like electrostatic field in a like gas substantially free from aerosols.

2. A system for determining the presence of aerosols in a gas, comprising two electrodes distanced and electrically insulated from each other and defining a space therebetween, a DC. voltage source applied to said electrodes and causing an electrostatic field in said space, means arranged adjacent said space for producing charged particles of one polarity in a predetermined portion of said space and causing a substantial unipolar current in said field, means connected with said electrodes for measuring said unipolar current, means for supplying said gas to said space, said aerosols when present in said gas accumulating to said charged particles of said one polarity and thereby decreasing said unipolar current to thereby afford a determination of the presence of said aerosols in said space by said measuring means.

3. A system for determining the presence of aerosols in a gas, comprising two electrodes distanced and electrically insulated from each other and defining a space therebetween, a DO. voltage source applied to said electrodes and causing an electrostatic field in said space, a radioactive substance emitting ionizing rays arranged adjacent to the first of said electrodes to produce charged particles in a first portion of said space, shield means arranged adjacent to said radioactive substance and between said substance and said second electrode to prevent said ionizing rays from reaching the second portion of said space and to thereby efiect a substantially unipolar current in said second portion of said space, means connected with said electrodes for measuring the current caused by said electrostatic field and said charged particles, means for supplying said gas to said space, said aerosols when present in said gas accumulating to said charged particles of said one polarity and thereby decreasing said unipolar current to thereby afiord a determination of the presence of said aerosols in said space by said measuring means.

4. A system according to claim 3, wherein said first portion of said space is smaller than said second portion thereof.

5. A system for determining the presence of aerosols in a gas, comprising two electrodes distanced and electrically insulated from each other and defining a space therebetween, the first electrode being of the jacket type,

the second electrode having the form of a rod and being centrally arranged in said first electrode, a DC. voltage source applied to said electrodes and causing an electrostatic field in said space, a ring centrally arranged in and connected with said first electrode, the axial length of said ring being substantially smaller than the axial length of said first electrode, and defining an outer and an inner portion in said space, a radioactive substance emitting ionizing rays being arranged at the outside of said ring, to thereby direct said ionizing rays into said outer portion of said space and to efiect a substantially unipolar current in the inner portion of said space, means connected with said electrodes for measuring the current caused by said electrostatic field and said charged particles, means for supplying said gas to said space, said aerosols when present in said gas accumulating to said charged particles of said one polarity and thereby decreasing said unipolar current to thereby afiord a determination of the presence of said aerosols in said space by said measuring means.

6. System according to claim 4, wherein said first electrode is provided with inlet and outlet openings for said gas.

7. A fire alarm device, comprising aerosol detecting means including two electrodes distanced and electrically insulated from each other and defining a space therebetween, a DC. voltage source having a negative and a positive pole, a relay and a resistor serially connected with said electrodes, and having junction points therebetween, a radioactive substance emitting ionizing rays arranged adjacent to the first of said electrodes, shield means defining a first and a second portion in said space, said shield means preventing said ionizing rays from reaching the second portion of said space to thereby effect a substantially unipolar current in said second portion of said space, a gas filled tube having a cathode, and anode and a control electrode, said cathode being connected with the negative pole of said D.C. voltage source, said anode being connected with the junction point between said serially connected resistor and said relay, said control electrode being connected with the junction point between said serially connected second electrode and said resistor, means for supplying said gas to said space, said aerosols when present in said gas in a predetermined value decreasing said current as to elfect an ignition of said gas filled tube and thereby actuating said relay.

8. Device according to claim 7, wherein said resistor is an ionization chamber having a radioactive substance emitting ionizing rays.

9. Device according to claim 7, wherein one of said electrodes is carried by said gas filled tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,051 Donelian Sept. 24, 1946 2,594,777 Hicks Apr. 29, 1952 2,702,898 Meili Feb. 22, 1955 2,785,312 Martin Mar. 12, 1957 

